A picosecond CO2 laser was used successfully in a number of experiments exploring advanced method... more A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm−3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser’s 10 μm wavelength shifts the critical plasma density to 1019 cm−3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, Jan 1, 2010
We report on the absolute calibration of a micro channel plate (MCP) detector, installed as detec... more We report on the absolute calibration of a micro channel plate (MCP) detector, installed as detector in a Thomson parabola spectrometer. The calibration delivers the relation between a registered count numbers in the CCD camera (on which the MCP phosphor screen is imaged) as a result of the impact of an ion beam onto the MCP. The particle response of the whole detection system was evaluated by using laser accelerated ions with proton energies up to 3 MeV and carbon ion energies up to 16 MeV. In order to obtain an absolute measurement of the number of ions incident on the MCP detector, slotted CR-39 track detector was installed in front of the MCP. The signal registered on the MCP due to ions propagating through the CR-39 slots is compared to the number of particles counted on the adjacent CR-39 stripes after the etching. The calibration of the response of MCP has been extended to higher energy ions and protons on the basis of a simple model validated by comparison with the calibration data. This sensitive detection set-up makes it possible to measure in a single laser shot the ion spectrum in absolute terms.
In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (1... more In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (10 10 ), ultrashort (50 fs) laser pulses focused to intensities of 7 × 10 20 W cm −2 is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is 50 nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higher energy ions than oblique incidence at 35 • with respect to the target normal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration. a number of desirable properties, including high brightness (up to 10 13 ions in a picosecond bunch at the source) and low transverse and longitudinal emittance (about 100-fold better than beams produced from typical RF accelerators) . The interest in laser-accelerated ions is driven by the potential compactness and lower cost of these sources compared to more conventional accelerators. Among the many possible applications of laser-generated high-energy ion beams are the fast ignition approach to inertial confinement fusion [6], ion beam radiography [7], nuclear physics and therapeutic medicine . The need to optimize and control this novel source of high-energy ions for applications is motivating a worldwide research effort on high-power laser-driven ion acceleration.
By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that... more By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam's betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotron-like x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield.
We report on experimental investigations of proton acceleration from thin foil targets irradiated... more We report on experimental investigations of proton acceleration from thin foil targets irradiated with ultra-short (∼50 fs), high contrast (∼1010) and ultra-intense (up to 1021 W/cm2) laser pulses. These measurements provided for the first time the opportunity to extend the scaling laws for the acceleration process in the ultra-short regime beyond the 1020 W/cm2 threshold. The scaling of accelerated proton energies was investigated by varying the thickness of Al targets (down to 50 nm) under 35° angle of laser incidence and with p-polarised light.
High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives... more High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives to conventional ion beam sources due to their unique properties such as high charge, low emittance, compactness and ease of beam delivery. Typically the acceleration is due to the rapid expansion of a laser heated solid foil, but this usually leads to ion beams with large energy spread. Until now, control of the energy spread has only been achieved at the expense of reduced charge and increased complexity. Radiation pressure acceleration (RPA) provides an alternative route to producing laser-driven monoenergetic ion beams. In this paper, we show the interaction of an intense infrared laser with a gaseous hydrogen target can produce proton spectra of small energy spread (~ 4%), and low background. The scaling of proton energy with the ratio of intensity over density (I/n) indicates that the acceleration is due to the shock generated by radiation-pressure driven hole-boring of the critical surface. These are the first high contrast mononenergetic beams that have been theorised from RPA, and makes them highly desirable for numerous ion beam applications.
A picosecond CO2 laser was used successfully in a number of experiments exploring advanced method... more A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm−3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser’s 10 μm wavelength shifts the critical plasma density to 1019 cm−3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, Jan 1, 2010
We report on the absolute calibration of a micro channel plate (MCP) detector, installed as detec... more We report on the absolute calibration of a micro channel plate (MCP) detector, installed as detector in a Thomson parabola spectrometer. The calibration delivers the relation between a registered count numbers in the CCD camera (on which the MCP phosphor screen is imaged) as a result of the impact of an ion beam onto the MCP. The particle response of the whole detection system was evaluated by using laser accelerated ions with proton energies up to 3 MeV and carbon ion energies up to 16 MeV. In order to obtain an absolute measurement of the number of ions incident on the MCP detector, slotted CR-39 track detector was installed in front of the MCP. The signal registered on the MCP due to ions propagating through the CR-39 slots is compared to the number of particles counted on the adjacent CR-39 stripes after the etching. The calibration of the response of MCP has been extended to higher energy ions and protons on the basis of a simple model validated by comparison with the calibration data. This sensitive detection set-up makes it possible to measure in a single laser shot the ion spectrum in absolute terms.
In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (1... more In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (10 10 ), ultrashort (50 fs) laser pulses focused to intensities of 7 × 10 20 W cm −2 is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is 50 nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higher energy ions than oblique incidence at 35 • with respect to the target normal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration. a number of desirable properties, including high brightness (up to 10 13 ions in a picosecond bunch at the source) and low transverse and longitudinal emittance (about 100-fold better than beams produced from typical RF accelerators) . The interest in laser-accelerated ions is driven by the potential compactness and lower cost of these sources compared to more conventional accelerators. Among the many possible applications of laser-generated high-energy ion beams are the fast ignition approach to inertial confinement fusion [6], ion beam radiography [7], nuclear physics and therapeutic medicine . The need to optimize and control this novel source of high-energy ions for applications is motivating a worldwide research effort on high-power laser-driven ion acceleration.
By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that... more By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam's betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotron-like x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield.
We report on experimental investigations of proton acceleration from thin foil targets irradiated... more We report on experimental investigations of proton acceleration from thin foil targets irradiated with ultra-short (∼50 fs), high contrast (∼1010) and ultra-intense (up to 1021 W/cm2) laser pulses. These measurements provided for the first time the opportunity to extend the scaling laws for the acceleration process in the ultra-short regime beyond the 1020 W/cm2 threshold. The scaling of accelerated proton energies was investigated by varying the thickness of Al targets (down to 50 nm) under 35° angle of laser incidence and with p-polarised light.
High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives... more High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives to conventional ion beam sources due to their unique properties such as high charge, low emittance, compactness and ease of beam delivery. Typically the acceleration is due to the rapid expansion of a laser heated solid foil, but this usually leads to ion beams with large energy spread. Until now, control of the energy spread has only been achieved at the expense of reduced charge and increased complexity. Radiation pressure acceleration (RPA) provides an alternative route to producing laser-driven monoenergetic ion beams. In this paper, we show the interaction of an intense infrared laser with a gaseous hydrogen target can produce proton spectra of small energy spread (~ 4%), and low background. The scaling of proton energy with the ratio of intensity over density (I/n) indicates that the acceleration is due to the shock generated by radiation-pressure driven hole-boring of the critical surface. These are the first high contrast mononenergetic beams that have been theorised from RPA, and makes them highly desirable for numerous ion beam applications.
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Papers by Nicholas Dover